Spatial and temporal variability in particle-bound pesticide exposure and their effects on benthic community structure in a temporarily open estuary

Spatial and temporal variations in particle-bound pesticide contamination, natural environmental variables, and benthic abundance were measured during the dry summer season within a temporarily open estuary (Lourens River). This study focused on the effect of particle-associated pesticides on the dynamics of the benthic community (including epi-benthic, hyper-benthic, and demersal organisms) by comparing two runoff events, differing in their change in pesticide concentration and environmental variables. The two chosen sites were situated within the upper and middle reaches of the estuary and differ significantly in salinity (p = 0.001), flow (p = 0.5), temperature (p < 0.001) and particulate organic carbon in the sediment (p < 0.001). Generally higher particle-bound pesticides were found in the upper reaches. rst runoff event was characterised by an increase in pesticides (chlorpyrifos, endosulfan and cypermethrin) and hardly any change in natural environmental variables, whereas the second runoff event was characterised by no increase in pesticide but a significant change in natural environmental variables like salinity, temperature and flow. The most evident spatial difference in community structure was shown by the use of Principal Response Curve after the first runoff event, whereas no response was shown after the second runoff event. The variables which explained most of the spatial differences are Total Organic Carbon, salinity, chlorpyrifos and endosulfan concentrations. The species contributing most to the spatial differences are the estuarine harpacticoid Mesochra and Canthocamptus (lower abundance at the upper reaches) and the freshwater species Dunhevedia and Thermocyclops (higher abundance within upper reaches). Within the spatial variability (between upper and middle reaches) the authors were able to detect a link between endosulfan, chlorpyrifos exposure, TOC and salinity and community change by comparing the two runoff events.